nathelper Module

Maxim Sobolev

Sippy Software, Inc.

Juha Heinanen

TuTPro, Inc.

Edited by

Maxim Sobolev

Edited by

Bogdan-Andrei Iancu

Edited by

Juha Heinanen

Edited by

Ovidiu Sas

Table of Contents

1. Admin Guide
1. Overview
2. NAT pinging types
3. Dependencies
3.1. Kamailio Modules
3.2. External Libraries or Applications
4. Parameters
4.1. force_socket (string)
4.2. natping_interval (integer)
4.3. ping_nated_only (integer)
4.4. natping_processes (integer)
4.5. natping_socket (string)
4.6. received_avp (str)
4.7. sipping_bflag (integer)
4.8. sipping_from (string)
4.9. sipping_method (string)
4.10. nortpproxy_str (string)
4.11. keepalive_timeout (int)
5. Functions
5.1. fix_nated_contact()
5.2. fix_nated_sdp(flags [, ip_address])
5.3. add_rcv_param([flag]),
5.4. fix_nated_register()
5.5. nat_uac_test(flags)
5.6. is_rfc1918(ip_address)
5.7. add_contact_alias([ip_addr, port, proto])
5.8. handle_ruri_alias()
6. Exported Pseudo Variables
6.1. $rr_count
6.2. $rr_top_count
7. MI Commands
7.1. nh_enable_ping
8. Selects
8.1. @nathelper.rewrite_contact[n]
2. Frequently Asked Questions

List of Examples

1.1. Set force_socket parameter
1.2. Set natping_interval parameter
1.3. Set ping_nated_only parameter
1.4. Set natping_processes parameter
1.5. Set natping_socket parameter
1.6. Set received_avp parameter
1.7. Set sipping_bflag parameter
1.8. Set sipping_from parameter
1.9. Set sipping_method parameter
1.10. Set nortpproxy_str parameter
1.11. Set keepalive_timeout parameter
1.12. fix_nated_contact usage
1.13. fix_nated_sdp usage
1.14. add_rcv_paramer usage
1.15. fix_nated_register usage
1.16. add_contact_alias usage
1.17. handle_ruri_alias usage
1.18. $rr_count usage
1.19. $rr_top_count usage
1.20. nh_enable_ping usage
1.21. @nathelper.rewrite_contact usage

Chapter 1. Admin Guide

1. Overview

This is a module to help with NAT traversal and reuse of tcp connections. In particular, it helps symmetric UAs that don't advertise they are symmetric and are not able to determine their public address.

Function fix_nated_contact() rewrites Contact header field with request's source address:port pair. Function fix_nated_sdp() adds the active direction ndication to SDP (flag 0x01) and updates source IP address too (flag 0x02). Function fix_nated_register() exports exports the request's source address:port into an AVP to be used during save() and should be used for REGISTER requests.

Note: fix_nated_contact changes the Contact header, thus it breaks the RFC. Although usually this is not an issue, it may cause problems with strict SIP clients. An alternative is to use add_contact_alias() that together with handle_ruri_alias() is standards conforming and also supports reuse of TCP/TLS connections.

Known devices that get along over NATs with nathelper are ATAs (as clients) and Cisco Gateways (since 12.2(T)) as servers. See">

2. NAT pinging types

Currently, the nathelper module supports two types of NAT pings:

  • UDP package - 4 bytes (zero filled) UDP packages are sent to the contact address.

    • Advantages: low bandwitdh traffic, easy to generate by Kamailio;

    • Disadvantages: unidirectional traffic through NAT (inbound - from outside to inside); As many NATs do update the bind timeout only on outbound traffic, the bind may expire and closed.

  • SIP request - a stateless SIP request is sent to the contact address.

    • Advantages: bidirectional traffic through NAT, since each PING request from Kamailio (inbound traffic) will force the SIP client to generate a SIP reply (outbound traffic) - the NAT bind will be surely kept open.

    • Disadvantages: higher bandwitdh traffic, more expensive (as time) to generate by Kamailio;

3. Dependencies

3.1. Kamailio Modules

The following modules must be loaded before this module:

  • usrloc module - only if the NATed contacts are to be pinged.

3.2. External Libraries or Applications

The following libraries or applications must be installed before running Kamailio with this module loaded:

  • None.

4. Parameters

4.1. force_socket (string)

Socket to be used when sending NAT pings for UDP communication. If no one specified, the OS will choose a socket.

Default value is NULL.

Example 1.1. Set force_socket parameter

modparam("nathelper", "force_socket", "localhost:33333")

4.2. natping_interval (integer)

Period of time in seconds between sending the NAT pings to all currently registered UAs to keep their NAT bindings alive. Value of 0 disables this functionality.


Enabling the NAT pinging functionality will force the module to bind itself to USRLOC module.

Default value is 0.

Example 1.2. Set natping_interval parameter

modparam("nathelper", "natping_interval", 10)

4.3. ping_nated_only (integer)

If this variable is set then only contacts that have behind_NAT flag in user location database set will get ping.

Default value is 0.

Example 1.3. Set ping_nated_only parameter

modparam("nathelper", "ping_nated_only", 1)

4.4. natping_processes (integer)

How many timer processes should be created by the module for the exclusive task of sending the NAT pings.

Default value is 1.

Example 1.4. Set natping_processes parameter

modparam("nathelper", "natping_processes", 3)

4.5. natping_socket (string)

Spoof the natping's source-ip to this address. Works only for IPv4.

Default value is NULL.

Example 1.5. Set natping_socket parameter

modparam("nathelper", "natping_socket", "")

4.6. received_avp (str)

The name of the Attribute-Value-Pair (AVP) used to store the URI containing the received IP, port, and protocol. The URI is created by fix_nated_register function of nathelper module and the attribute is then used by the registrar to store the received parameters. Do not forget to change the value of corresponding parameter in registrar module if you change the value of this parameter.


You must set this parameter if you use "fix_nated_register". In such case you must set the parameter with same name of "registrar" module to same value.

Default value is "NULL" (disabled).

Example 1.6. Set received_avp parameter

modparam("nathelper", "received_avp", "$avp(i:42)")

4.7. sipping_bflag (integer)

What branch flag should be used by the module to identify NATed contacts for which it should perform NAT ping via a SIP request instead if dummy UDP package.

Default value is -1 (disabled).

Example 1.7. Set sipping_bflag parameter

modparam("nathelper", "sipping_bflag", 7)

4.8. sipping_from (string)

The parameter sets the SIP URI to be used in generating the SIP requests for NAT ping purposes. To enable the SIP request pinging feature, you have to set this parameter. The SIP request pinging will be used only for requests marked so.

Default value is NULL.

Example 1.8. Set sipping_from parameter

modparam("nathelper", "sipping_from", "")

4.9. sipping_method (string)

The parameter sets the SIP method to be used in generating the SIP requests for NAT ping purposes.

Default value is OPTIONS.

Example 1.9. Set sipping_method parameter

modparam("nathelper", "sipping_method", "INFO")

4.10. nortpproxy_str (string)

The parameter sets the SDP attribute used by nathelper to mark the packet SDP informations have already been mangled.

If empty string, no marker will be added or checked.


The string must be a complete SDP line, including the EOH (\r\n).

Default value is a=nortpproxy:yes\r\n.

Example 1.10. Set nortpproxy_str parameter

modparam("nathelper", "nortpproxy_str", "a=sdpmangled:yes\r\n")

4.11. keepalive_timeout (int)

The parameter sets the interval in secods after which a natted contact is removed from location table if it does not reply to SIP keepalives (usually OPTIONS ping requests).

The features is available only for UDP contacts that are stored in memory (not working for db only mode for usrloc module).

Keepalives are sent stateless, not using TM module. The value of this parameter has to be few times higher than natping_interval.

Default value is 0 (feature disabled).

Example 1.11. Set keepalive_timeout parameter

modparam("nathelper", "keepalive_timeout", 120)

5. Functions

5.1.  fix_nated_contact()

Rewrites Contact HF to contain request's source address:port.

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE.

Example 1.12. fix_nated_contact usage

if (search("User-Agent: Cisco ATA.*") {fix_nated_contact();};

5.2.  fix_nated_sdp(flags [, ip_address])

Alters the SDP information in orer to facilitate NAT traversal. What changes to be performed may be controled via the flags parameter. Return value is -1 if error occurred, 1 if ip's were replaced, 2 if no ip's were replaced.

Meaning of the parameters is as follows:

  • flags - the value may be a bitwise OR of the following flags:

    • 0x01 - adds a=direction:active SDP line;

    • 0x02 - rewrite media IP address (c=) with source address of the message or the provided IP address (the provide IP address take precedence over the source address).

    • 0x04 - adds a=nortpproxy:yes SDP line;

    • 0x08 - rewrite IP from origin description (o=) with source address of the message or the provided IP address (the provide IP address take precedence over the source address).

  • ip_address - IP to be used for rewritting SDP. If not specified, the received signalling IP will be used. The parameter allows pseudo-variables usage. NOTE: For the IP to be used, you need to use 0x02 or 0x08 flags, otherwise it will have no effect.


Example 1.13. fix_nated_sdp usage

if (search("User-Agent: Cisco ATA.*") {fix_nated_sdp("3");};

5.3.  add_rcv_param([flag]),

Add received parameter to Contact header fields or Contact URI. The parameter will contain URI created from the source IP, port, and protocol of the packet containing the SIP message. The parameter can be then processed by another registrar, this is useful, for example, when replicating register messages using t_replicate function to another registrar.

Meaning of the parameters is as follows:

  • flag - flags to indicate if the parameter should be added to Contact URI or Contact header. If the flag is non-zero, the parameter will be added to the Contact URI. If not used or equal to zero, the parameter will go to the Contact header.

This function can be used from REQUEST_ROUTE.

Example 1.14. add_rcv_paramer usage

add_rcv_param(); # add the parameter to the Contact header
add_rcv_param("1"); # add the parameter to the Contact URI

5.4.  fix_nated_register()

The function creates a URI consisting of the source IP, port, and protocol and stores the URI in an Attribute-Value-Pair. The URI will be appended as "received" parameter to Contact in 200 OK and registrar will store it in the received cloumn in the location table.

Note: You have to set the received_avp parameter of the nathelper module and the registrar module (both module parameters must have the same value) to use this function.

This function can be used from REQUEST_ROUTE.

Example 1.15. fix_nated_register usage


5.5.  nat_uac_test(flags)

Tries to guess if client's request originated behind a nat. The parameter determines what heuristics is used.

Meaning of the flags is as follows:

  • 1 - Contact header field is searched for occurrence of RFC1918 addresses.

  • 2 - the "received" test is used: address in Via is compared against source IP address of signaling

  • 4 - Top Most VIA is searched for occurrence of RFC1918 addresses

  • 8 - SDP is searched for occurrence of RFC1918 addresses

  • 16 - test if the source port is different from the port in Via

  • 32 - test if the source IP address of signaling is a RFC1918 address

  • 64 - test if the source connection of signaling is a WebSocket

  • 128 - test if the Contact URI port differs from the source port of the request (Warning: this is might be legal or even intended combination in non natted scenarios)

All flags can be bitwise combined, the test returns true if any of the tests identified a NAT.


5.6.  is_rfc1918(ip_address)

Determines if the address in the parameter is an rfc1918 address. The parameter allows pseudo-variables usage.

This function can be used from ANY_ROUTE.

5.7.  add_contact_alias([ip_addr, port, proto])

Adds ;alias=ip~port~transport parameter to contact URI containing either received ip, port, and transport protocol or those given as parameters.


Example 1.16. add_contact_alias usage

    if (!is_present_hf("Record-Route")) {
        if (!add_contact_alias("$var(src_ip)", "$Rp", "tcp")) {
            xlog("L_ERR", "Error in aliasing contact $ct\n");
            send_reply("400", "Bad request");

5.8.  handle_ruri_alias()

Checks if Request URI has alias param and if so, removes it and sets $du based on its value. Note that this means that routing of request is based on ;alias parameter value of Request URI rather than Request URI itself. If you call handle_ruri_alias() on a request, make thus sure that you screen alias parameter value of Request URI the same way as you would screen Request URI itself.

Returns 1 if ;alias param was found and $du was set and $ru rewritten, 2 if alias param was not found and nothing was done, or -1 in case of error.

This function can be used from REQUEST_ROUTE, BRANCH_ROUTE, and LOCAL_ROUTE.

Example 1.17. handle_ruri_alias usage

    if ($du == "") {
        switch ($rc) {
        case -1:
            xlog("L_ERR", "Failed to handle alias of R-URI $ru\n");
            send_reply("400", "Bad request");
        case 1:
            xlog("L_INFO", "Routing in-dialog $rm from $fu to $du\n");
        case 2:
            xlog("L_INFO", "Routing in-dialog $rm from $fu to $ru\n");

6. Exported Pseudo Variables

6.1. $rr_count

Number of Record Routes in received SIP request or reply.

Example 1.18. $rr_count usage

    $avp(rr_count) = $rr_count;

6.2. $rr_top_count

If topmost Record Route in received SIP request or reply is a double Record Route, value of $rr_top_count is 2. If it a single Record Route, value of $rr_top_count is 1. If there is no Record Route(s), value of $rr_top_count is 0.

Example 1.19. $rr_top_count usage

    if ($rr_count == $avp(rr_count) + $rr_top_count) {

7. MI Commands

7.1. nh_enable_ping

Enables natping if parameter value greater than 0. Disables natping if parameter value is 0.

The function takes only one parameter - a number in decimal format.

Example 1.20. nh_enable_ping usage

$ kamctl fifo nh_enable_ping 1

8. Selects

8.1. @nathelper.rewrite_contact[n]

Get n-th Contact value with IP:Port rewritten to source ip:port. N is counted from 1. Only IP:port is rewritten, remaining part are left unchanged. Full nameaddr is supported.

Example 1.21. @nathelper.rewrite_contact usage

$c = @nathelper.rewrite_contact[1];
$c2 = @nathelper.rewrite_contact[1].nameaddr.uri;

Chapter 2. Frequently Asked Questions

2.1. What happend with “rtpproxy_disable” parameter?
2.2. Where can I find more about Kamailio?
2.3. Where can I post a question about this module?
2.4. How can I report a bug?


What happend with rtpproxy_disable parameter?

It was removed as it became obsolete - now rtpproxy_sock can take empty value to disable the rtpproxy functionality.


Where can I find more about Kamailio?

Take a look at


Where can I post a question about this module?

First at all check if your question was already answered on one of our mailing lists:

E-mails regarding any stable Kamailio release should be sent to and e-mails regarding development versions should be sent to .

If you want to keep the mail private, send it to .


How can I report a bug?

Please follow the guidelines provided at: